Photographic elements made predominantly of silver chloride, with minor amounts of silver bromide and iodide, are well-known in the prior art. These elements have wide processing latitude and can be made and utilized for most of the art fields which employ silver halide as the sensitive medium. High-chloride emulsions offer the advantages of greater solubility (allowing for faster development and fixing times), and lower native sensitivity to visible light (ideal for color applications, among others) compared to other photographically useful silver halides. However, since sensitized silver chloride elements are much slower than those containing mainly bromide, their use has been generally limited to graphic arts applications, e.g., contact, low-speed camera films, and the like. It would be desirable to combine the high-speed characteristics of bromide-rich photographic emulsions with the rapid and convenient processibility of chloride-rich emulsions, a combination which is needed in many silver halide art fields.
Tabular grain silver halide products are known in the prior art and present the user with some considerable advantages over conventional grain products, e.g., those products having semi-spheroidal grains. The tabular products exhibit higher covering power, can be more effectively spectrally sensitized, are more easily developed and can tolerate a higher level of hardening without loss in covering power, each providing quite an advantage over the conventional grains.
Tabular chloride emulsions are also known and are described by Wey in U.S. Pat. No. 4,399,215, and by Maskasky, U.S. Pat. No. 4,400,463. These prior art tabular chloride emulsions are, however, not as advantageous to use since they are limited generally to large, thick tabular grains and require the use of binder supplements other than gelatin. For example, in the aforementioned Wey patent, a process for preparing extremely large, thick tabular silver chloride elements is described. The Wey process uses ammonia as a crystal growth agent and the grains produced have little utility in commercial applications. The Maskasky patent teaches the use of both a growth modifying amount of an aminoazaindene and asynthetic peptizer containing a thioether linkage, and is also limited to the preparation of large tubular silver chloride elements.
There is a need to prepare a suitable tabular grain silver halide emulsion having good speed and processing latitude, wherein at least 50 mole percent of the grains of this emulsion are chloride and are photographically useful, the emulsion being prepared without the use of a synthetic peptizer.